update-test-checks: safely handle tests with #if's

There is at least one Clang test (clang/test/CodeGen/arm_acle.c) which
has functions guarded by #if's that cause those functions to be compiled
only for a subset of RUN lines.

This results in a case where one RUN line has a body for the function
and another doesn't. Treat this case as a conflict for any prefixes that
the two RUN lines have in common.

This change exposed a bug where functions with '$' in the name weren't
properly recognized in ARM assembly (despite there being a test case
that was supposed to catch the problem!). This bug is fixed as well.

Differential Revision: https://reviews.llvm.org/D130089